1. Technical Field
Some example embodiments of the present invention generally relate to an image forming apparatus and/or a fixing device, for example, for fixing a toner image on a recording medium, e.g., by induction heating.
2. Description of Background Art
A background image forming apparatus, for example, a copying machine, a facsimile machine, a printer, or a multifunction printer having copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data by an electrophotographic method. For example, a charger charges a surface of a photoconductor. An optical writer emits a light beam on the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to image data. The electrostatic latent image is developed with a developer (e.g., toner) to form a toner image on the photoconductor; The toner image is transferred from the photoconductor onto a sheet. A fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet. Thus, the toner image is formed on the sheet.
One example of a background fixing device uses induction heating to shorten a time period needed for the fixing device to be heated up to a proper fixing temperature after being powered on, so as to save energy. The fixing device includes a magnetic flux generator including a coil, a fixing roller including a heat generating layer, and/or a pressing roller: The magnetic flux generator opposes a part of an outer circumferential surface of the fixing roller. The pressing roller pressingly contacts another part of the outer circumferential surface of the fixing roller to form a fixing nip. At the fixing nip, the fixing roller and the pressing roller apply heat and pressure to a sheet bearing a toner image conveyed to the fixing nip to fix the toner image on the sheet. The coil extends in a width direction (i.e., a direction perpendicular to a sheet conveyance direction) of the magnetic flux generator.
For example, a power source applies a high-frequency alternating current to the coil to form an alternating magnetic field around the coil. An eddy current generates in the heat generating layer. An electric resistance of the heat generating layer generates Joule heat. The Joule heat increases the temperature of the whole fixing roller. Induction heating may heat the fixing roller up to a desired temperature in a shortened time period by consuming less energy compared to heating with a heating lamp, for example.
Another example of a background fixing device includes a magnetic flux generator, a pressing roller, and/or a fixing roller. The magnetic flux generator is disposed inside the pressing roller. The fixing roller contacts the pressing roller, and includes a temperature-sensitive, magnetic metal pipe. A member including a non-magnetic material (e.g., aluminum) having a low electric resistivity is disposed inside the temperature-sensitive, magnetic metal pipe. The temperature-sensitive, magnetic metal pipe includes a magnetic shunt alloy providing self-control of temperature. Thus, in this example fixing device, induction heating may effectively heat the fixing roller.
Yet another example of a background fixing device includes a fixing roller including a heat generating layer having various layer thicknesses in a width direction of the heat generating layer (i.e., a width direction of the fixing roller). For example, a layer thickness of a center portion of the heat generating layer in the width direction of the heat generating layer is greater than a layer thickness of both end portions of the heat generating layer in the width direction of the heat generating layer: Thus, the fixing device may provide a proper width of the fixing nip which may prevent faulty fixing.
The above-described background fixing devices may perform faulty fixing due to a varied temperature distribution in the width direction of the fixing roller. For example, both end portions of the fixing roller in the width direction of the fixing roller dissipate heat in a greater amount than a center portion of the fixing roller in the width direction of the fixing roller Especially during a warm-up period of the fixing device when the fixing device is powered on after a long time period has elapsed since the fixing device was powered off, the fixing device is heated from a relatively low temperature up to a proper fixing temperature. Accordingly, the amount of dissipated heat substantially differs between the both end portions and the center portion of the fixing roller in the width direction of the fixing roller. Namely, the temperature of the both end portions of the fixing roller is lower than the temperature of the center portion of the fixing roller in the width direction of the fixing roller.